1. Field of the Invention
The present invention relates to image display devices and, more particularly, to an image display device in which a light emitting element is used in a pixel.
2. Description of the Prior Art
As an image display device employing light emitting elements for pixels, an EL display using electro-luminescence (hereinafter abbreviated to EL) elements has been reported. Besides, in an active matrix type EL display, wiring lines for signal and current transmission are formed in the shape of a matrix and pixel circuits are respectively built into pixels, wherein each pixel circuit is formed of thin-film transistors (hereinafter abbreviated to TFTs) which are active elements besides a light emitting element.
There are two methods of controlling the light emitting brightness of an EL element: a method in which voltage supplied to the EL element through the pixel circuit is controlled and a method in which current supplied to the EL element is controlled. Since the light emitting brightness of the EL element changes in proportion to the current flowing across the EL element, the method in which the current is controlled is advantageous in that it can provide stable control of the light emitting brightness. Such a method of controlling the light emitting brightness of an EL element by controlling the current flowing across the EL element is disclosed in JP2000-56847A.
A conventional pixel circuit equipped with an EL element is shown in FIG. 13. This pixel circuit of prior art is comprised of a resistor 101, p-channel TFTs 102 and 103, a TFT switch 104, a power source line 105, and a capacitor 106, and an EL element 108 and a ground electrode 107 are connected to the pixel circuit. When the TFT switch 104 is turned on and a voltage signal is applied to an input terminal 109, current flows across the resistor 101 and, at the gate electrode of a p-channel TFT 102, a gate voltage depending on a drain current is generated. The gate voltage is held on the capacitor 106. Current i that flows at this time is obtained by Equation 1 below, where Vdd is a voltage on the power source line 105, Vin is a voltage supplied to the input terminal 109, Vds is a drain-source voltage of the TFT 102, and R is a resistance value of the resistor 101.i=(Vdd−Vds−Vin)/R  (Equation 1)
Because the p-channel TFTs 102 and 103 constitute a current mirror circuit, the current i also occurs between the source and drain electrodes of the p-channel TFT 103 and also flows into the EL element 108. Then, even if the TFT switch 104 is turned OFF, the p-channel TFT 103 continues to supply the current i to the EL element 108, independent of the voltage at the input terminal 109, because the capacitor 106 holds the gate voltage of the TFT 103.
Thus, the pixel circuit shown in FIG. 13 is able to flow the current obtained by Equation 1 into the EL element 108 by controlling the voltage Vin supplied to the input terminal and, besides, is able to keep the current flowing across the EL element 108 by the gate voltage held by the capacitor 106. Since the light emitting brightness of the EL element 108 is proportional to the current flowing across the EL element 108, the light emitting brightness of the EL element 108 can be controlled by controlling the voltage Vin supplied to the input terminal. A huge number of pixel circuits identical to the above pixel circuit including the EL element are arrayed in two dimensions and the signal voltage Vin is input in order to the input terminal of each pixel circuit; thereby, an image can be displayed. As the EL element whose light emitting brightness changes in proportion to the quantity of the current flowing across it, an organic EL diode is known.